DOCS-1461 - Add accessibility guidelines to contributing docs

[kimsauce]

Purpose of this pull request

Adds a new accessibility.md page to the Contributing section covering WCAG 2.1 AA standards, alt text, link text, heading hierarchy, color contrast, ableist language, tables, animated content, and screen reader testing guidance.

Select the type of change

• Minor Changes - Typos, formatting, slight revisions
• Update Content - Revisions, updating sections
• New Content - New features, sections, pages, tutorials
• Site and Tools - .clabot, version updates, maintenance, dependencies, new packages for the site (Docusaurus, Gatsby, React, etc.)

Ticket (if applicable)

https://sumologic.atlassian.net/browse/DOCS-1461

[kimsauce]

Folding accessibility content into the style guide as a new ## Accessibility section on PR #7089 (DOCS-482) instead. Closing this draft.